A plastic film, which has been roughened on its surface for imparting an improved writing property thereto, has essential disadvantages, such as blotting of ink, roughness of particles on a written area, difficulty of correcting or erasing the writings, etc., and therefore must be improved in its writing property by incorporating thereinto a roughening agent of fine powder. For example, Japanese Patent 242157 and Japanese Patent Publication 9167/59 describe a plastic film provided with a coating layer comprising a roughening agent and a binder; Japanese Patent Publication 37494/72 discloses a tracing film which is extruded after incorporating a dispersant and a roughening agent into a styrene resin; and British Patent 822894 discloses a coated polyester film in which the polyester film is provided with a polyisocyanate coating on which is formed an organic hydrophilic film containing a roughening agent. Such films incorporated with the roughening agent may be used for blueprinting by utilizing a light reflected from the surface.
On the other hand, a transparent film having writing property without the roughening agent may be used not only for blueprinting due to transmitted light but also for so called PS make-up. For example, Japanese Patent Publication No. 20595/71 describes a method of forming a writable rough surface by utilizing an orange peel phenomenon, or applying a special solution of vinylidene chloride/acrylic ester compolymer onto a polyester film, which is then dried.
It has now been found, after having studied a transparent base film having an excellent writing property without using a conventional specific resin and a roughening agent, that roughening treatment of a polyester film by sand-blasting and formation of a film layer of styrene-butadiene rubber thereon may impart an excellent writing property to the film layer free of intrinsic writability, resulting in several advantages in that the writing property is distinct from that of the original rough surface, that particles of a pencil writing become fine, that deposition and absorption of ink may be good and that a clear picture may be achieved. It has further been found that the advantages just described may be also achieved by using, in lieu of the styrene-butadiene rubber, a rubber material selected from natural rubber, butadiene rubber (BR), isoprene rubber (IR), chloroprene rubber (CR), isobutylene-isoprene rubber (IIR), acrylonitrile-butadiene rubber (NBR), ethylene-propylene-diene rubber (EPDR) and acrylic rubber. It has not been determined why the excellent writing property is achieved. Probably, transfer of the rough surface of the polyester film to the film layer of the rubber material, as well as effects of elasticity and frictional coefficient of the rubber material may produce the unexpected writing property.